Cloned DNA of the yeast cycl gene is being used in various ways to study the structure and regulation of this and other cytochrome c genes. The nucleotide sequence has been determined for all the DNA in cyc1, for 250 base pairs out the 5' end and for 280 base pairs out the 3' end. The cyc1 gene sequence is continuously complementary to that inferred from the yeast iso-1-cytochrome c protein sequence; there is no intervening sequence. The coding section in cycl has been used as a hybridization probe to detect lambda-yeast hybrid clones containing the related cyc7 gene, which codes for iso-2-cytochrome c. Sequencing of the leader-promoter region DNA of cyc7 and comparison with the analogous cyc1 sequence may provide a molecular explanation for the 20-fold difference in gene product levels for cyc1 and cyc7. By the use of yeast plasmid DNA as a vector, cloned cyc1 DNA and DNA from the nearby SUP4 gene will be introduced into recipient yeast strains. If expression of the plasmid-borne genes is observed, in vitro deletions will be made by DNA cutting and splicing, then transformation with DNAs deleted to various extents. By these experiments, we intend to determine the extent of the cyc1 and SUP4 transcription units.